With the rapid development of combination bipolar/complementary metal-oxide-semiconductor (BiCMOS) integrated circuitry, it has been noted in the art that the buried doped layers used to form the collectors of bipolar transistors may be used as buried source/drains in MOS transistors with a vertical gate extending from the buried layer to a surface doped layer. Contact from the surface to the buried layer is then made thus providing a vertical field effect transistor. The motivation for providing a vertical field effect transistor is that the surface area utilized by the transistor may be minimized while maintaining a gate length (i.e., the distance from the source to the drain) which will minimize punch-through, leakage and hot electron problems. Thus, a compact and robust structure may be fabricated. However, contact to the buried layers are usually achieved by forming a doped layer from the surface to the buried layer. The fabrication of a doped layer of this depth, tends to provide a great deal of lateral diffusion and thus does not provide a compact structure, in terms of integrated circuit surface area. Thus, the space savings gained by providing a vertical transistor is lost by providing complete functionality for this vertical transistor.
One method for providing a contact to the buried layer which requires less area is shown in Eklund, et al., U.S. patent application Ser. No. 178,728, filed Apr. 7, 1988 and assigned to the assignee of this application. In Eklund, et al. a trench is formed in the surface of an integrated circuit extending down to the buried contact. An insulating layer is formed on the sidewalls of the trench and the bottom part of the insulating layer is opened by anisotropic etching. The trench is then filled with conductive material. This process is incompatible with the formation of vertical gates because the gate oxide in the trench for the vertical gates must remain in tact. Therefore, in the prior art, complete separate processing steps for forming vertical gates and trench contacts must be provided.